Medical imaging is a powerful tool to assist in the performance of various types of medical operations, and to enhance diagnostics and other decisions concerning a medical procedure. Several systems are known in the art to produce real-time medical images via a variety of different imaging modalities. In instances of imaging of anatomical structures that involve periodic motion or cyclic phases, the imaging system may encompass a timing element to take this into account. Monitoring devices, such as an electrocardiogram (ECG) machine, are regularly incorporated into medical imaging systems to provide timing information for registering the captured images with respect to organ phase. Typically, the timing information obtained from such monitoring devices is not completely accurate. As well, the signal received from such monitoring devices involves a delay, and is not obtained in real-time with respect to the actual organ motion. Finally, an external monitoring device adds a cumbersome element to an already complex system.
U.S. Pat. No. 5,577,502 to Darrow et al entitled “Imaging of interventional devices during medical procedures”, is directed to a method for compensation of subject motion during tracking of an invasive device within the body of the subject. A device tracking unit determines the location of the invasive device relative to a fixed reference point. An imaging device acquires a reference image of the subject. A position detection means placed within the imaging device measures the location over time of a reference point of the subject. Each acquired image is stored together with the corresponding location of subject reference point. A subject tracking unit receives location information over time from position detection means, and computes translation and rotation movement of the subject from time of image acquisition to time of device location measurement. A registration unit receives the reference image, the net position and orientation change of the subject, and the device location. The registration unit translates and rotates the reference image to match the position and orientation of subject at the time of device location measurement. An image of the device is superimposed upon the translated/rotated image of the subject at its absolute position and orientation. The registration unit may also adjust the displayed location of the device, rather than the display of the image.
In addition to translation and rotation motion, the registration unit accounts for expansion and contraction of the subject, occurring due to a periodic motion cycle, such as the respiratory or cardiac cycles. Position detection means measures the change in the subject due to expansion and contraction, and feeds this information to the registration unit. The registration unit distorts the reference image in accordance with the expansion and contraction, and subsequent translation and rotation, thereby dynamically registering the image of the subject with the current device location.
Alternatively, the imaging device may monitor subject motion by obtaining subsequent projections of the subject and then detecting offset and cross-sectional size of the subject in these images. Further alternatively, a series of reference images are each gated to the periodic motion cycle. The imaging device acquires a series of images at different times within the cardiac cycle, as measured by an ECG signal. An ECG provides a signal for each measurement of device location. At a given time, an image from the series which corresponds to the ECG signal is selected as the reference image. The registration unit translates and rotates this reference image, with respect to the information received from the subject tracking unit for that time. A representation of the measured location of the device is superimposed upon the updated image, resulting in a registered image of the subject and the invasive device.
U.S. Pat. No. 5,622,174 to Yamazaki entitled “Ultrasonic diagnostic apparatus and image displaying system”, is directed to a system for determining movement velocities of a moving internal organ and providing a color display of the movement velocities over time. An ultrasonic probe transmits an ultrasound beam toward the heart. The transmitted ultrasonic beam is partially reflected by tissues of the heart as an echo signal and returned to the probe. The echo signal has a Doppler shift in frequency due to the Doppler effect. The echo signal is transduced into a voltage signal and supplied to a reception signal processor. The signal is beam-formed, detected and output to a B-mode digital scan converter (DSC). The B-mode DSC converts the image data of the signal to standard television scanning data, which is sent to an image synthesizer. The B-mode DSC also stores a plurality of image data at arbitrary cardiac timing in a B-mode frame memory.
The reception signal processor also sends the transduced echo signal to a phase detector. The phase detector performs phase detection on the Doppler shift frequency to extract the Doppler shift signal. A low pass filter filters out unnecessary Doppler signals resulting from valve motion or blood flow, leaving only the Doppler signal from the cardiac muscle. A frequency analyzer calculates physical values relating to the velocities at each sampling volume of a scan plane, using the Fast Fourier Transform (FFT) or auto-correlation method. These values include mean Doppler frequencies (corresponding to mean velocities of movement of the organ), variance (turbulence factors of Doppler spectrum), and maximum values of Doppler shift frequencies (maximum velocities of organ movement at sampling volume). These values are sent as color Doppler information to a vector-velocity calculator, which calculates the absolute movement velocities of the organ at each sampling volume point. A display presents the magnitude and/or direction of velocities, in accordance with a color scheme assignment. It is noted that detected ECG signals of the heart are used to trigger the signal generator (output reference pulses for transmission/reception of ultrasonic beams). In addition, these cardiac timing signals are used to produce a real-time image displaying changes in movement velocities of the heart in color.
U.S. Pat. No. 6,246,898 to Vesely et al entitled “Method for carrying out a medical procedure using a three-dimensional imaging and tracking system”, is directed to a method for performing an in-vivo medical procedure on an associated body using three-dimensional tracking and imaging. A plurality of transceivers is used to track the three-dimensional motion of an object under investigation. At least four transceivers are implanted within a specimen in whom distances are to be measured. Three transceivers lie in a (x,y) plane and act as a reference. The fourth transceiver determines the z-coordinates of surrounding transducers by determining if an active one of the transducers lies above or below the reference plane established by the three transceivers. Each of a plurality of transmitters attached to the specimen at various locations is sequentially fired, while the three reference transceivers record the receiver signals. Since the difference from each transmitter to the reference plane is known, the relative x,y,z coordinates of the transmitters can be determined using triangulation.
The video display is synchronized with the real-time patient heart beat using an ECG signal. An Analog-to-Digital (ND) converter converts the ECG signal into digital data. A sync generator module produces a timing signal corresponding to the current heart activity from the digital ECG data. This is done by activating a memory location or input port, or generating an interrupt, at the precise time a QRS complex is identified. In particular, the sync generator module tests the data signal for large rates of change, zero crossings, and other information allowing the sync generator module to reveal the QRS complex of the signal
U.S. Pat. No. 6,556,695 to Packer et al entitled “Method for producing high resolution real-time images, of structure and function during medical procedures”, is directed to a method for providing medical images in real-time to assist physicians in the performance of medical procedures. A Medical Resonance Imaging (MRI) system obtains a high resolution model of the heart prior to the medical procedure. Images of the heart are acquired during successive cardiac phases. A pulse generator creates a series of fast gradient echo pulse sequences. The R-R interval of the cardiac cycle is divided up into several short segments of pulse sequences, using an ECG gating signal that triggers at the peak of the R wave. A single coordinate, or view, of the heart is acquired during each fast gradient echo segment. Adjacent segments are combined into groups, and the data in each group contributes to generating an image at a different phase of the cardiac cycle. A number (e.g., fifteen) of two-dimensional slices are acquired during an entire cardiac cycle, depicting one slice through the heart at (e.g., fifteen) successive phases of the cardiac cycle. Additional slices of the heart are acquired and reconstructed into two-dimensional images. These two-dimensional slices are then combined to form (e.g., fifteen) three-dimensional image data sets.
During the medical procedure, an ultrasonic transducer acquires low-resolution image frames of the heart in real-time. An ECG signal from the patient detects the real-time cardiac phase. The stored high-resolution heart model is registered using the real-time image frames and ECG signal. The registered high-resolution model is then used to produce high-resolution, large field of view images in real-time on a display.